PROJECT SUMMARY/ABSTRACT Loneliness and social isolation are related, but distinct, social risk factors critical to the health and quality of life of older adults in their last years of life. Loneliness is a subjective feeling of being alone, whereas social isolation is an objective loss in the number of relationship with family, friends, or the community. Through separate pathways, each social risk factor is associated with poor quality of life, different health care use, functional impairment, and mortality. Older adults may be particularly vulnerable to loneliness and social isolation and their effects in the last years of life due to late life disability, cognitive impairment, and physical symptoms such as pain. These social risk factors may create barriers to accessing high quality end of life health care including hospice use and lead to difficulty in avoiding burdensome care such as hospitalizations or emergency department visits. In addition, loneliness is an often unrecognized distressing symptom in the last years of life and avoiding loneliness and social isolation in the last years of life is a high priority for older adults. The objective of this study is to establish the epidemiology of loneliness and social isolation in the last years of life of older adults and determine their association with health care utilization. Using a U.S. nationally- representative cohort of 2,282 older adults who died within two years of responding to questions about loneliness and social isolation we investigate two specific aims: 1) to determine the prevalence and predictors of a) loneliness and b) social isolation during the last two years of life, and 2) to determine the relationship between loneliness and social isolation with health service use at the end of life using Medicare claims data. This proposal will have a significant impact in encouraging clinicians caring for older adults at the end of life to recognize and treat social isolation and loneliness in the same way they prioritize treating symptoms like pain or dyspnea. Future directions of this work include designing targeted interventions to loneliness and social isolations in the last years of life, and addressing barriers to accessing adequate end-of-life care in the socially vulnerable.